(1) Field of the Invention
The present invention relates to a set of swashplates for controlling the pitch of rotor blades, and to a rotor and to an aircraft having such a set.
(2) Description of Related Art
A main rotor of a rotorcraft serves to provide the rotorcraft with lift and possibly also propulsion. Conventionally, such a main rotor comprises a rotor mast that rotates a plurality of blades via a rotor hub.
In order to control the movement of the rotorcraft, the pitch of the blades of the main rotor is controlled by a pilot.
Cyclic variation of the pitch of the blades serves to vary the pitch of each blade as a function of its azimuth angle. In contrast, collective variation of the pitch of the blades causes an identical variation in the pitch of all of the blades.
Under such circumstances, the pitch of the blades can be adjusted by means of pitch rods and by means of a set of swashplates surrounding the rotor mast.
In principle, the set of swashplates comprises a rotary swashplate connected to the pitch rods and a non-rotary swashplate connected to the flight controls.
The non-rotary swashplate is situated under the rotary swashplate or is surrounded by the rotary swashplate. Furthermore, the non-rotary swashplate is connected to the rotary swashplate in order to impart its movements to the rotary swashplate. Thus, the rotary swashplate follows all of the movements of the non-rotary swashplate and transmits those movements to the blades via the pitch rods.
Consequently, the non-rotary and rotary swashplates are suitable for being moved in translation along the rotor mast and they are also suitable for being tilted relative to the rotor mast, e.g. with the help of a plurality of servo-controls connected to the structure of the rotorcraft and to the non-rotary swashplate. As a general rule, the movements in translation and in tilting of the non-rotary and rotary swashplates are implemented with the help of a device including a mast ball joint centered on the axis of rotation of the rotor and slidable along a stationary structural element surrounding said rotor mast.
The non-rotary swashplate is then arranged in oscillating manner on the mast ball joint so as to be capable of being tilted and of being moved in translation relative to the rotor mast.
Furthermore, the non-rotary swashplate is secured to the structure of the rotorcraft with the help of at least one stationary scissors linkage that consequently prevents it from rotating about the axis of rotation of the rotor.
The rotary swashplate of the set of swashplates is then connected to the non-rotary swashplate by a member making the following possible:
the rotary swashplate can be constrained together with the non-rotary swashplate to move in translation along the rotor mast;
the rotary swashplate can oscillate about the mast ball joint together with the non-rotary swashplate; and
the rotary swashplate can rotate about the axis of rotation of the rotor together with the blades.
To this end, the rotary swashplate is provided with at least one rotary scissors linkage connected to the rotor mast, possibly via the hub of the main rotor. The rotor mast then acts either directly or indirectly to drive the rotary swashplate in rotation about the axis of rotation.
A scissors linkage, whether rotary or non-rotary, generally comprises two arms that are hinged together. More precisely, each scissors linkage comprises a primary arm and a secondary arm that are hinged together by a first hinge. The first hinge allows the two arms of a scissors linkage to move apart or towards each other so as to allow the swashplate connected to the scissors linkage to move in translation.
The primary arm is also connected via a second hinge to the rotor mast or to the structure of the rotorcraft, as appropriate, while the secondary arm is connected to the corresponding swashplate via a third hinge.
Although effective, said first, second, and third hinges are subject to wear requiring maintenance actions that are onerous and frequent.
In order to remedy that, scissors linkages are often over-dimensioned, and are therefore heavy and expensive to fabricate.
Furthermore, scissors linkages are large in size because of the movements they need to perform. Such large size can generate aerodynamic disturbances.
Thus, Document FR 2 768 996 seeks to replace the rotary scissors linkage by means that are lightweight, that are inexpensive to fabricate, and that generate less aerodynamic drag.
The set of swashplates then has drive means as an alternative to the rotary scissors linkage and provided with two rigid arms that are diametrically opposite, each rigid arm being secured both to the rotor mast and to the rotary swashplate via point connection means.
More precisely, each arm is provided with a U-shaped section defining a rigid drive track in which a finger of the rotary swashplate is received. In combination, the finger and the rigid track then constitute the point connection means.
In order to co-operate with the rigid drive track, the finger includes a wheel, a pair of wheels, a sliding skid, or indeed a sliding skid and a wheel, depending on the variant.
Document FR 2 951 699 describes a set of swashplates. That set includes drive means provided with a drive arm and annular linear connection means secured to a rotary swashplate rotatable about an axis of rotation. A first end of the drive arm is connected to the rotary swashplate by the annular linear connection means and a second end of the drive arm is connected to a rotor mast by slideway connection means of the drive means.
Also known is Document EP 2 679 493.